1. Field of the Invention
The present invention relates to light-emitting diodes (LED) drivers, or more particularly, to a system and method for preventing automatic dropout in an LED driver if an output current (or voltage) cannot be maintained.
2. Description of Related Art
Light-emitting diodes (LEDs) can be found in many different types of technologies, including flashlights, Christmas lights, and electronic devices. For example, LEDs are commonly used as backlights (e.g., edge-lit, array, etc.) for electronic devices having a liquid crystal display (LCD) (e.g., laptop computers, PDAs, cellular telephones, MP3 players, etc.).
When LEDs are used in electronic devices, steps are generally taken to ensure that the LEDs are provided with sufficient power. For example, an LED voltage driver might be used to provide a threshold voltage (e.g., 3.6 volts) to an LED. A problem arises, however, when a battery source for the electronic device is unable to provide the required threshold voltage. This may be due to the battery source being depleted, either as a result of time or the application of a load.
If the output voltage of the LED driver drops too low, it can cause individual LEDs to appear dim or inactive. This is because each LED has its own electrical characteristics. For example, a first LED may draw 20 mA at 3.3 volts, whereas a second LED may draw 15 mA at 3.3 volts. Because current in an LED is directly proportional to an amount of light produced, low voltage can result in an LED backlight having a non-uniform appearance (i.e., certain LEDs appear brighter than others). Also, if the voltage drop is due to a temporary load, then the LEDs may appear as if they are blinking (e.g., dimming when a load is applied, and increasing when the load is removed).
One way of solving this problem is to use a circuit (e.g., a capacitive charge pump, an inductive boost, etc.) to increase voltage. However, such a circuit adds cost to the device, and may require a relatively large package. Thus, in a world where devices are generally smaller and less expensive, many manufactures are reluctant to add such a circuit.
Another way of solving this problem is to monitor the LED voltage driver, and power down the electronic device if the output voltage of the LED voltage driver drops below a threshold voltage (e.g., 3.6 volts). While this ensures that the LED backlight is uniform when it is functional, it results in a shortened battery life (or at least an appearance of a shortened battery life), since the device is powered down after the voltage threshold can no longer be maintained.
Accordingly, it would be advantageous to provide a circuit that would prevent an automatic dropout in an LED driver if an output current (or voltage) cannot be maintained. Such a circuit, for example, would allow the device (e.g., portable electronic device, etc.) to continue to run at a reduced and regulated current (or voltage), thereby producing a light source even if an initial output current (or voltage) cannot be maintained.